the 3,6 m indo-belgian devasthal optical … · the 3.6 m indo-belgian devasthal optical te...

The 3.6 m Indo-Belgian Devasthal Optical Telescope: Performance results on site

Nathalie Ninane, Christian Bastin, Carlo Flebus and Brijesh Kumar*

Advanced Mechanical and Optical Systems (AMOS s.a.), LIEGE Science Park, B-4031 ANGLEUR (Liège), BELGIUM

* Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital, 263 129 India

ABSTRACT

AMOS SA has been awarded of the contract for the design, manufacturing, assembly, tests and on site installation (Devasthal, Nainital in central Himalayan region) of the 3.6 m Indo-Belgian Devasthal Optical Telescope (IDOT). The telescope has Ritchey-Chrétien optical configuration with one axial and two side Cassegrain ports. The meniscus primary mirror is active and it is supported by pneumatic actuators. The azimuth axis system is equipped with hydrostatic bearing. After successful factory acceptance at AMOS SA, the telescope has been dismounted, packed, transported, and re-mounted on site. This paper provides the final performances (i.e. image quality, pointing and tracking) measured during sky tests at Devasthal Observatory. Keywords: telescope tests, active optics, image quality, pointing, tracking, IDOT

1. INTRODUCTION After telescope design(1), AIV and factory acceptance tests(2) performed in AMOS assembly hall, IDOT was dismounted and packed to be sent to its site, Devasthal in India. End of 2014, the building was ready for telescope remounting and commissioning. Few months later, first light, and fine tuning could start. And finally, performance and acceptance tests were conducted end of 2015.

After a short telescope overview and requirement discussion, some acceptance test results are presented.

2. TELESCOPE OVERVIEW The main characteristics of the telescope are summarized in Table 1. As shown in Figure 1, the optical combination is a Ritchey-Chrétien type with a Cassegrain focus where the light beam can be directed toward a main port designed for interfacing an instrument with a mass up to 2 tons or toward two side ports for smaller instruments. The mount type is an alt-azimuth. The telescope weights 150 tons. It rotates around the azimuth axis thanks to an hydraulic track(3). The telescope is equipped with an active optic system(4) (AOS) that controls the primary mirror figure and the secondary mirror positioning to keep the telescope wavefront error in the specification for any operational conditions. The primary mirror is a meniscus 165 mm thick, 3700 mm diameter supported by 69 axial actuators and 24 lateral astatic levers. While an Acquisition and Guiding Unit (AGU) aligned on a guide star at the edge of the telescope field of view measures the wavefront and tracking errors, the set of forces applied by the actuators to the mirror is adjusted continuously. In parallel, the Telescope Control System(5) (TCS) computes the telescope trajectory taking into account of the weather conditions, the pointing model of the telescope and the tracking errors measured by the AGU.

Ground-based and Airborne Telescopes VI, edited by Helen J. Hall, Roberto Gilmozzi, Heather K. Marshall, Proc. of SPIE Vol. 9906, 99064E · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2232775

Proc. of SPIE Vol. 9906 99064E-1

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Figure 2 shows pictures of the telescope on site. The telescope sizes are: height 13 m, width 7 m and total weight 150 tons. The telescope is installed at Devasthal, North of India, at an altitude of 2540 m. A picture of the dome is in Figure 3; it is a rotating cylinder equipped with as large slit and a wind screen. Large fans all around enable the venting and limit the dome seeing.

Figure 1: ARIES telescope and ray tracing showing the side port and axial port optical path; with 1) the primary mirror, 2) the secondary, 3) the side port focal plane, 4) the side port folding mirror, 5) the field corrector and 6) the axial focal plane.

Type: Ritchey - Chrétien Focal length: 32.4 m (telescope F#/9 with M1 F#/2) Aperture stop: 3.6 m on M1 2 focal plane configurations: Side Port and Axial Port Field of View: 10 arcmin on side ports,

30 arcmin on axial port, (35 arcmin for the AGU)

Operational waveband: 350 nm to 5000 nm M1 characteristics: R = 14638.87 mm CC

K = -1.03296 Optical Φ = 3600 mm, mechanical Φ = 3700 mm

M2 characteristics: R = -4675.30 mm CX K = -2.79561 Optical Φ = 952 mm, mechanical Φ = 980 mm

Distance M1-M2: 5.51 m Back focal length: 2.5 m Focal plane Radius of curvature: 1935.78 mm Scale plate: 157 µm / arcsec (0.006 arcsec/µm)

Table 1: Summary of the telescope characteristics

1

2 3

4 5

6



°

Figure 2: IDOT on site

Figure 3: IDOT building

3. REQUIREMENTS AND REQUIREMENT VERIFICATIONS Table 2 summarizes the main performance requirements specified for 3.6 m IDOT.

Optical main requirements Image quality - Encircled Energy 50% < 0.3 arcsec,

- Encircled Energy 80% < 0.45 arcsec, - Encircled Energy 90% < 0.6 arcsec, For the waveband 350 nm to 1500 nm; without corrector for 10 arcmin FOV.



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Figure 10: Main port pointing test; at the left the star sample in the telescope field of regard, at the right the dispersion of the star

centroids while sighting them with the telescope.

The tracking error is specified in open and closed loop of the guider. The tracking error is measured in recording the star image on a CCD camera in the telescope focal plane while tracking. The statistics on the movements of the centroids give the tracking error. Centroid positioning were measured with integration time of 30 sec. The measurements were repeated several times for covering the particular axis movements that appear in the field of regard of the telescope. Close to zenith, azimuth and rotator axes accelerate and rotate larger angles in a given lapse of time; close to Polaris the axes are nearly still or can change their moving direction. Close to horizon the seeing becomes worst, good measurements are then difficult to catch. Except for measurements close to horizon the tracking requirements were met. The discrepancy at horizon is obviously due to the higher seeing present there. Typical test results in open and close loop tracking are given in figure 11 and 12.

Figure 11: Open loop tracking test; the star trajectory while measuring at the left, at the right the plot in blue gives the RMS error for data acquired during 1 minute, the plot in green is the peak deviation of the centroid for sliding elapsed time of 15 minutes.



Telescope trajectory in Azimuth - Altitude coordinatesMeasure date: 2015_12_2

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Figure 12: Close loop tracking test; the star trajectory while measuring at the left, at the right the plot in blue gives the RMS error for data acquired during 1 minute, the plot in green is the RMS deviation of the centroid for sliding elapsed time of 1 hour.

6. CONCLUSION IDOT commissioning and acceptance test campaign happened successfully despite of the difficult context. Indeed, the specifications of the telescope are written such that the performance is limited by the seeing and the customer is willing an end to end acceptance test. The first difficulty is to transform theoretical specifications into measurable parameters in observing conditions. These parameters have to be sufficiently reliable despite of the seeing and in accordance with the customer final need. The image quality requirements expressed in Encircled Energy was transformed in wavefront error specification. The customer who are astronomers are not used to work with this criteria and would like to get an independent way to prove the image quality with a measurement related to image resolution. Fortunately some good seeing nights enabled to demonstrate the accordance of what was measured with the wavefront sensor and the telescope quality in acquiring double star images.

It has to be reminded that a telescope is a complex machine with some very accurate sub-systems and with finely tuned controls. The obtained quality is the result of an attentive work on every sub-system.

Before sending the telescope on site, it was completely mounted, tuned and tested in the factory integration hall (2). Despite of the fact that the hall seeing was bad (2 arc seconds in the best case), the debugging and experience acquired at that time were very precious on site. Time was spared and all the attention could be put on the fine tuning and the alignments that are necessary for reaching the specifications.

IDOT is the first 4-m class telescope equipped with an active primary mirror realized by AMOS; its success is very encouraging for the new projects granted to AMOS; DKIST Primary mirror cell(6) and the Doğu Anadolu Gözlemevi (DAG) telescope(7) that uses this demonstrated technology.

7. ACKNOWLEDGEMENT This work has been performed under ARIES contract reference 1985-14-02. AMOS is very grateful towards ARIES team for having put their confidence in AMOS team for the design and manufacturing of the 3.6 m telescope.



At the end of the commissioning and test campaign, ARIES large field camera was installed at the focus of the telescope. This gave us (ARIES and AMOS team) the opportunity to acquire a first “astronomic” image. Thanks for having spent this moment together. The image is presented in Figure 13 and was acquired during the first working night of the telescope Imager instrument (which was thus not tuned nor aligned at that time). Seeing during this night was quite bad (~1.5 arcsec).

Figure 13: Crab Nebula; composition of 2 images (R and V bands); acquired with IDOT and ARIES large field camera.

8. REFERENCES

[1] Ninane N., Flebus C. and Kumar B., "The 3.6 m Indo-Belgian Devasthal Optical Telescope: general description",

Proc. SPIE 8444-67 (2012) [2] Ninane N., Bastin C., Deville J., Michel F., Pierard M., Gabriel E., Flebus C. and Omar A., "The 3.6 m Indo-Belgian

Devasthal Optical Telescope: assembly, integration, and tests at AMOS", Proc. SPIE 8444-102 (2012). [3] Deville J., Bastin C. and Pierard M., "The 3.6 m Indo-Belgian Devasthal Optical Telescope: the hydrostatic azimuth

bearing", Proc. SPIE 8444-150 (2012) [4] Pierard M., Schumacher J.M., Flebus C. and Ninane N., "The 3.6 m Indo-Belgian Devasthal Optical Telescope: the

active M1 mirror support", Proc. SPIE 8444-186 (2012) [5] Gabriel E., Bastin C., Piérard M., "The 3.6 m Indo-Belgian Devasthal Optical Telescope: the control system", Proc.

SPIE 8451-82 (2012) [6] Gregory P. Lousberg, Vincent Moreau, Jean-Marc Schumacher, Maxime Piérard, Aude Somja, Pierre Gloesener,

Carlo Flebus, “Design and analysis of an active optics system for a 4-m telescope mirror combining hydraulic and pneumatic supports”, Proc. SPIE 9626-24 (2015)

[7] Grégory P. Lousberg, Emeric Mudry, Christian Bastin, Jean-Marc Schumacher, Eric Gabriel, Olivier Pirnay, Carlo Flebus, “Active optics system for the 4m telescope of the Eastern Anatolia Observatory (DAG)”, Proc. SPIE 9912-223 (2016)



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